System and method for reducing windage losses in compressor motors

ABSTRACT

A method and system for reducing windage losses in compressor motors is provided. The compressor motor is cooled by circulating refrigerant from a closed refrigerant loop incorporating the compressor. A pumping device coupled to a liquid expander in the closed refrigerant loop circulates refrigerant through the motor cavity and produces a motor cavity pressure lower than evaporating pressure. The lower pressure in the motor cavity reduces the density of the gasses in the motor cavity, resulting in reduced windage losses of the motor. Additionally, the pumping device is powered by the recovered liquid expansion energy between the condenser and the evaporator.

BACKGROUND OF THE INVENTION

The present invention relates to a system and method of cooling acompressor motor by circulating refrigerant gas over the motorcomponents. More specifically, the present invention is directed toreducing windage losses in a compressor motor by lowering the pressureand density of the refrigerant gas within the motor cavity.

High-speed motors typically have large windage losses, in part becauseof the large amount of cooling gas induced windage friction causedduring high-speed rotor rotation, which impacts the motor's performanceand efficiency. To reduce the windage losses, factors directly relatedto the motor such as the peripheral speed of the rotor, the flow ofmotor cooling gas around the motor, the rotor surface area and theroughness of the rotor surface are manipulated and controlled tooptimize the performance of the motor.

One method for reducing energy losses in motors while cooling the motoris by suctioning refrigerant toward the motor windings. The reduction intemperature of the motor windings prevents the motor components fromoverheating and creates more operating efficiency. Another method forreducing energy losses in motors is to maintain constant pressurethroughout the motor cavity. A pressure valve can be placed within themotor cavity to release higher-pressure gas build up that occurs in themotor cavity during operation. As the pressure in the cavity increases,the valve opens, thereby releasing high-pressure gases. The maintenanceof constant pressure in the cavity increases motor efficiency. However,this method uses mechanical equipment and is not optimal for maintaininga true constant pressure in the motor cavity. Additionally, this methoddoes not address the issue of the motor cavity temperature.

An additional method controls energy losses in motors by maintaining aconstant pressure in the motor cavity, while also preventing the oillosses between motor components. The preservation of oil in the motorbearing components allows for greater lubrication for the movement ofparts thereby reducing friction while not allowing oil to escape intothe motor cooling cavity, preventing excessive oil churning and reducingenergy losses. A hermetically sealed housing containing therefrigeration compressor transmission and oil supply reservoir isconnected to the suction side of the compressor to equalize the pressurein the housing. The focus of the method is to prevent the boiling ofrefrigerant from the oil reserve. However, this system only holds thepressure in the motor cavity at a constant level, and only assists inreducing energy losses, rather than optimizing the motor efficiency.

For very high speed motors however, windage losses can still besubstantial even after factors such as the peripheral speed of therotor, the density and flow of motor cooling gas around the motor, therotor surface area and/or the roughness of the rotor surface areoptimized. The only remaining factor that can be manipulated to reducewindage losses is the density of the gas in the motor cavity. Windagelosses decrease as the density of the gas in the motor cavity decreasesresulting in better motor efficiency.

To reduce the gas density in these higher-speed motor cavities, vacuumpumps are used to lower the pressure surrounding the motors to reducewindage losses as much as possible. However, these systems lack theability to both adequately cool the motor while providing a vacuumsurrounding the motor cavity. One attempt to lower the gas density inthe motor cavity while simultaneously cooling the motor involves the useof auxiliary positive displacement gas compressors powered by anindependent power source to “pump down” the motor cavity while acomplete chiller system is in operation. However in these systems, theauxiliary compressors consume more energy than they are saving in motorwindage losses, therefore these systems are not a good solution toincreasing motor efficiency.

Therefore, there is a need for a system that can reduce windage andother energy losses in a compressor motor while not expending moreenergy than is being saved.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a refrigerationsystem including a compressor, an evaporator and a condenser connectedin a closed refrigerant loop. A motor is connected to the compressor toprovide power to the compressor. A liquid expander is connected in therefrigerant loop between the condenser and the evaporator. Inconjunction with the refrigeration system, a motor coolant system isused to cool the compressor motor. The motor coolant system has a firstconnection with the refrigerant loop to receive refrigerant from theevaporator to the motor cavity for cooling, and a second connection withthe refrigerant loop to return refrigerant to the evaporator from themotor cavity. The motor coolant system also has a pumping device tocirculate refrigerant from the first connection through the motor cavityand to the second connection. The pumping device is powered by operationof the liquid expander and the pumping device lowers the pressure anddensity of the gaseous refrigerant in the motor cavity to reduce windagelosses in the motor.

A second embodiment of the present invention is directed to a motorcoolant system for a chiller system including a compressor, anevaporator and a condenser connected in a closed refrigerant loop. Themotor coolant system includes a motor housing for a motor that powersthe compressor of the chiller system. The motor coolant system alsoincludes a liquid expander that is connectable in the closed refrigerantloop between the condenser and the evaporator of the chiller system.Additionally, the motor coolant system has a first connectionconnectable to the closed refrigerant loop to receive refrigerant fromthe evaporator and provide refrigerant to the motor housing and a secondconnection connectable to the closed refrigerant loop to returnrefrigerant to the evaporator. A pumping device is disposed in thesecond connection and is used to circulate refrigerant from the firstconnection through the motor housing to the second connection to coolthe motor and maintain a predetermined pressure in the motor cavity. Thepumping device is coupled to a liquid expander and is powered byoperation of the liquid expander. Further, the predetermined pressure inthe motor cavity is maintained at a constant level throughout theoperation of the motor coolant system.

Another embodiment of the invention is a method for cooling a motor of achiller system including the steps of providing a first connect with arefrigerant loop, where the first connection is configured to receiverefrigerant from an evaporator. The next step involves providing asecond connection with the refrigerant loop, where the second connectionis configured to return refrigerant to the evaporator, and thenproviding a motor in a motor cavity, where the motor cavity is connectedto the first connection and the second connection. The next stepinvolves circulating refrigerant from the first connection through themotor cavity to the second connection with a pumping device, and thenpowering the pumping device with energy of expansion from a liquidexpander, where the liquid expander is configured to expand refrigerantin the refrigerant loop between a condenser and the evaporator, whereinthe circulation of refrigerant in the motor cavity by the pumping devicecools the motor and lowers the pressure and gas density of a refrigerantin the motor cavity thereby reducing windage losses of the motor.

One advantage of the present invention is the reduction in windage andenergy losses in the motor.

Another advantage of the present invention is the recycling ofdischarged energy by the liquid expander.

Still another advantage of the present invention is that the systemeffectively lowers the pressure of refrigerant gas in the motor cavity,cools the motor, and keeps energy expenses at a minimum. All thisoptimizes the reduction of windage losses and increases the efficiencyof the motor.

Additionally, another advantage of the present invention is that thecompressor for the motor cooling loop is load dependant. Therefore, thesystem only operates at the necessary level for the current load of thesystem and does not consume unnecessary energy.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of another embodiment of the presentinvention.

FIG. 3 illustrates a cross section of a motor and compressor housing.

FIG. 4 illustrates a detailed view of the connection between the pumpingdevice and the expander.

DETAILED DESCRIPTION OF THE INVENTION

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. Referring to FIG. 1,the HVAC, refrigeration or liquid chiller system includes a compressor302, a condenser arrangement 112, and a liquid chilling evaporatorarrangement 114 connected in a refrigerant loop. In a preferredembodiment, the chiller system has a capacity of 250 tons or greater andeven more preferably, has a capacity of 1000 tons or greater. A motor106 is connected to the compressor 302 to power the compressor 302. Themotor 106 and compressor 302 are preferably housed in a common hermeticenclosure, but can be housed in separate hermetic enclosures. Thecompressor 302 compresses a refrigerant vapor and delivers high pressurevapor to the condenser 112 through a discharge line. The compressor 302is preferably a centrifugal compressor; however, the compressor 302 canbe any suitable type of compressor including a screw compressor, areciprocating compressor, a scroll compressor, a rotary compressor orany other type of compressor.

The high pressure refrigerant vapor delivered by the compressor 302 tothe condenser 112 enters into a heat exchange relationship with a fluid,such as air or water, and undergoes a phase change to a high pressurerefrigerant liquid as a result of the heat exchange relationship withthe fluid. The high pressure liquid refrigerant from the condenser 112flows through an expander 128 to enter the evaporator 114 at a lowerpressure. The liquid refrigerant delivered to the evaporator 114 entersinto a heat exchange relationship with a fluid, e.g., air or water, andundergoes a phase change to a refrigerant vapor as a result of the heatexchange relationship with the fluid. The vapor refrigerant in theevaporator 114 exits the evaporator 114 and returns to the compressor302 by a suction line to complete the cycle. It is to be understood thatany suitable configuration of condenser 112 and evaporator 114 can beused in the system, provided that the appropriate phase change of therefrigerant in the condenser 112 and evaporator 114 is obtained.

A motor cooling loop is connected to the refrigerant loop discussedabove to provide cooling to the motor 106. The motor cooling loop has aconnection near the suction inlet of the compressor 302 that leads tothe motor cavity of the motor 106. The circulated refrigerant gas forcooling the motor 106 exits the motor cavity and is sent to theevaporator 114. As discussed in greater detail with regard to FIGS. 3and 4, a pumping device 130 is used to circulate the refrigerant throughthe motor cavity from the refrigerant loop near the suction inlet of thecompressor 302 and return the refrigerant to the refrigerant loop nearthe evaporator 114. The circulation of the refrigerant from therefrigerant loop into the motor cavity and the removal of the heatedrefrigerant gas from the motor cavity by the pumping device 130 helps tocool and lower windage losses in the motor 106 and raise the overallmotor efficiency. In particular, the operation of the pumping device 130is used to maintain a substantial constant predetermined pressure anddensity of refrigerant gas in the motor cavity to lower windage losses.The predetermined pressure and density of refrigerant gas in the motorcavity is less than the suction pressure of the compressor and canapproach a vacuum type condition. The HVAC or refrigeration system caninclude many other features that are not shown in FIG. 1. These featureshave been purposely omitted to simplify the drawing for ease ofillustration.

Similar to FIG. 1, FIG. 2 also has a compressor 302, a condenser 112,and an evaporator 114 connected in a closed refrigerant loop. Thecompressor 302 compresses the refrigerant vapor and delivers highpressure vapor to the condenser 112 through a discharge line. The highpressure refrigerant vapor delivered to the condenser 112 enters into aheat exchange relationship with a fluid from a cooling tower, e.g.,water, and undergoes a phase change to a high pressure refrigerantliquid as a result of the heat exchange relationship with the fluid. Thehigh pressure liquid refrigerant from the condenser 112 flows throughthe expander 128 and enters the evaporator 114 at a lower pressure. Theevaporator 114 includes connections for a supply line and a return lineof a cooling load. A secondary liquid, e.g., water, ethylene glycol,calcium chloride brine or sodium chloride brine, travels into theevaporator 114 via a return line and exits the evaporator 114 via asupply line for a cooling load. The liquid refrigerant in the evaporator114 enters into a heat exchange relationship with the secondary liquidto lower the temperature of the secondary liquid. The refrigerant liquidin the evaporator 114 undergoes a phase change to a refrigerant vapor asa result of the heat exchange relationship with the secondary liquid.The vapor refrigerant in the evaporator 114 exits the evaporator 114 andreturns to the compressor 302 by a suction line to complete the cycle.

As in FIG. 1, the motor cooling loop is connected to the refrigerantloop to provide cooling to the motor 106. The motor cooling loop has aconnection near the suction inlet of the compressor 302 that leads tothe motor cavity for the motor 106. However, different from theembodiment in FIG. 1, the circulated motor coolant refrigerant gas,after cooling the motor 106 and passing through the pumping device 130,is passed through a heat exchanger 134 to lower the temperature of thesuperheated refrigerant gas before the refrigerant gas is sent to theevaporator 114. The heat exchanger 134 has a connection with the supplyline between the cooling tower 132 and the condenser 112 to receivecooling water from the cooling tower 132. Water from the cooling tower132 is used to cool the refrigerant gas exiting the pumping device 130,by de-superheating the refrigerant as it flows through heat exchanger134. After the cooling water exchanges heat with the refrigerant, thecooling water is returned to the cooling tower 132 with a connection tothe return line between the condenser 112 and the cooling tower 132. TheHVAC or refrigeration system can include many other features that arenot shown in FIG. 2. These features have been purposely omitted tosimplify the drawing for ease of illustration.

As shown in both FIGS. 1 and 2, the pumping device 130 is coupled to theexpander 128 from the refrigerant loop. The pumping device is preferablya compressor, and can be any one of a screw compressor, a reciprocatingcompressor, a scroll compressor, a vane type compressor or othersuitable compressor. For example, in a 1000 ton capacity chiller system,the pumping device or compressor 130 preferably has a swept volume of atleast about 310 CFM and a volume ratio of at least about 3.3 to deliverthe necessary pressures. The pumping device 130 and the expander 128 canbe mechanically coupled via a common shaft, or by having two separatemechanical components that are tied together electrically where theexpander 128 is coupled to a type of electric generator, and the pumpingdevice 130 is powered by an electric motor that uses the requiredportion of the electric that is generated. The pumping device 130 andthe expander 128 can also be integrated into a single system unit havingeither a mechanical or electrical connection with a common shaft. Asingle system unit utilizes a control valve to control or limit theamount of expander power extraction so that the depressed pressure inthe motor cavity can be controlled. In utilizing a control valve, theexcess expansion refrigerant is essentially expanded through a part ofthe slide control orifice to satisfy the cooling load liquid refrigerantflow requirements into the evaporator. With the single system unithaving the pumping device 130 and the expander 128 with a control valveto regulate motor cavity pressure and control expansion of the liquidrefrigerant, only four refrigerant connections are required on anefficient chiller component with no shaft seals. Whenpositive-displacement compression technology is used for the pumpingdevice 130 and the expander 128, the required pressure ratios and volumeratios are attainable. If aerodynamic compression technology isutilized, the required pressure ratios and volume ratios are achievedthrough the incorporation of additional aerodynamic stages on thepumping device 130 and/or the expander 128 to achieve the requiredpressure ratios and volume ratios for proper operation. Preferably, theexpander 128 is one of an eductor, a positive displacement expander, orturbine type centrifugal expander. For example, in a 1000 ton capacitychiller system, the expander 128 preferably is sized for at least 300GPM liquid refrigerant inlet flow with a volume ratio of at least about13.8 to fully expand the liquid as needed for the system. It is to beunderstood that the particular swept volume and volume ratio minimumsfor the expander 128 and pumping device 130 are dependant on a varietyof factors such as the type of refrigerant used and the capacity of therefrigeration system. The expander 128 provides power to the pumpingdevice 130 by recovering the discharged energy from the expansion of theliquid refrigerant. The use of recovered energy to power the pumpingdevice 130 reduces energy losses of the motor coolant system and alsoreduces the amount of total power needed to operate the motor coolantsystem.

In addition, the connection of the pumping device 130 to the expander128 permits the operation of the motor coolant system to be loaddependant. When the load on the motor is reduced, the motor operates ata lower speed and can have a corresponding reduced cooling demand.Additionally, at lower load capacity, the coupled pumping device 130receives less power from the expander 128 due to reduced flow ofrefrigerant through the primary refrigerant loop and the pumping devicecorrespondingly provides a lower amount of suction on the motor cavityto siphon off refrigerant gasses cooling the motor 106. Since the systemis load dependant, it never reduces the gas density of the refrigerantin the motor cavity lower than necessary or expends more energy thannecessary.

As shown in FIG. 3, an aerodynamic compressor 302 is powered by ahermetic motor 106. The compressor 302 can be any one of a single stagecompressor, or a multiple-stage compressor configured on a common shaftwith the motor 106, or with the motor 106 disposed between the multiplestages. The motor 106 includes a stator 502 having a plurality ofprojecting poles (i.e. motor windings), and a rotor 504 also having aplurality of poles. In the cross-sectional drawing in FIG. 3, there areshown only one pair of poles for each of the stator 502 and the rotor504, although the motor 106 normally had multiple pole-pairs on each ofthe stator 502 and the rotor 504. The stator 502 typically has a greaternumber of poles than the rotor 504. The rotor 504 is attached to a shaft508 that is connected to and drives the impeller 510 of the compressor302. A plurality of electrical connectors 518 connects the poles of thestator 502 to impart rotation to the rotor 504 and the impeller 510. Themotor 106 is shown within the hermetic enclosure 516 that encloses thecompressor 302 and its associated components.

The motor 106 and motor cavity are maintained at a pressure much lowerthan the suction pressure of the compressor 302 at the suction line 524to reduce windage losses. The motor 106 and motor cavity are in fluidcommunication with the suction line 524 and the compressor chamber 528via conduit 526 (shown schematically in FIG. 3). The conduit 526 is influid communication with motor passages 530 that exist between the rotor504 and the stator 502. The refrigerant gas inside the motor 106 isdrawn from the compressor chamber 528 into the motor passages 530thereby circulating refrigerant vapor inside the motor 106 and motorcavity to cool the motor 106. The now heated refrigerant gas is drawnfrom the motor cavity by the pumping device 130 and then sent to theheat exchanger 134 and/or the evaporator 114 by the pumping device 130.

Referring to FIG. 4, a cross sectional illustration of one connectionbetween the expander 128 and the pumping device 130 is shown. Theexpander 128 and the pumping device 130 are shown connected by amechanical connection. The expander 128 and the pumping device 130operate on a common shaft, where the expander 128 drives the compressor130 based on the amount of refrigerant from the condenser 112 flowingthrough the expander 128. The pumping device 130 receiving gassesdirectly from the motor cavity, and the expander 128 receives liquidrefrigerant from the condenser 112. The pumping device 130 transfers thedischarged motor gas to the heat exchanger 134 and/or the evaporator114. The expander 128 uses the excess energy from the expansion of therefrigerant to power the pumping device 130. As the expander 128processes the excess energy, the energy is transferred to the connectedpumping device 130, thereby supplying power to the pumping device 130.The refrigerant is then discharged from the expander 128 to theevaporator 114 before returning to the compressor 302.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A refrigeration system comprising: a compressor, an evaporator and acondenser connected in a refrigerant loop; a motor connected to thecompressor to power the compressor, the motor being disposed in a motorcavity; a liquid expander connected in the refrigerant loop between thecondenser and the evaporator; and a motor coolant system, the motorcoolant system comprising: a first connection with the refrigerant loopto receive refrigerant from the evaporator; a second connection with therefrigerant loop to return refrigerant to the evaporator; a pumpingdevice to circulate refrigerant from the first connection through themotor cavity to the second connection, the pumping device being poweredby operation of the liquid expander; and wherein the pumping devicelowers a pressure and gas density of the refrigerant in the motor cavityto reduce windage losses of the motor.
 2. The refrigeration system ofclaim 1 wherein the liquid expander is configured to expandhigh-pressure refrigerant liquid from the condenser to low-pressurerefrigerant liquid for the evaporator.
 3. The refrigeration system ofclaim 2 wherein the liquid expander powers the pumping device byrecovering energy from the expansion of refrigerant in the liquidexpander.
 4. The refrigeration system of claim 3 wherein the liquidexpander comprises one of an eductor, a positive displacement expanderor a turbine centrifugal expander.
 5. The refrigeration system of claim1 wherein the pumping device is a gas compressor.
 6. The refrigerationsystem of claim 5 wherein the gas compressor comprises one of anaerodynamic compressor or a positive displacement compressor.
 7. Therefrigeration system of claim 6 wherein the gas compressor comprises oneof a screw compressor, a reciprocating compressor, a scroll compressor,or a vane type compressor.
 8. The refrigeration system of claim 7,wherein the refrigeration system has as a 1000 ton capacity, the gascompressor has a swept volume of about 310 CFM and a volume ratio ofabout 3.3, and the liquid expander is configured for a flow of at least300 GPM and has a volume ratio of about 13.8.
 9. The refrigerationsystem of claim 1 wherein the liquid expander is coupled to the pumpingdevice by one of a mechanical connection or an electrical connection.10. The refrigeration system of claim 1 wherein the liquid expander andthe pumping device are combined as a single unit.
 11. The refrigerationsystem of claim 1 further comprising a heat exchanger connected betweenthe pumping device and the evaporator, the heat exchanger beingconfigured to de-superheat refrigerant discharged from the pumpingdevice.
 12. The refrigeration system of claim 11 wherein the heatexchanger is configured to de-superheat refrigerant from the pumpingdevice with condenser cooling tower water.
 13. A motor coolant systemfor a chiller system having a compressor, an evaporator and a condenserconnected in a closed refrigerant loop, the motor coolant systemcomprising: a motor housing for the motor; a liquid expander connectableto the closed refrigerant loop between the condenser and the evaporatorof the chiller system; a first refrigerant connection connectable to theclosed refrigerant loop to receive refrigerant from the evaporator andprovide refrigerant to the motor housing; a second refrigerantconnection connectable to the closed refrigerant loop to returnrefrigerant to the evaporator; and a pumping device disposed in thesecond refrigerant connection to circulate refrigerant from the firstrefrigerant connection through the motor housing to the secondrefrigerant connection to cool the motor and maintain a predeterminedpressure in the motor housing, the pumping device being coupled to theliquid expander and powered by operation of the liquid expander.
 14. Themotor coolant system of claim 13 wherein the predetermined pressure inthe motor housing is maintained at a predetermined level throughout theoperation of the motor coolant system.
 15. The motor coolant system ofclaim 13 wherein the coupled pumping device and liquid expander areconnected by one of a mechanical connection or an electrical connection.16. The motor coolant system of claim 13 wherein the coupled pumpingdevice and liquid expander are connected as one unit.
 17. The motorcoolant system of claim 13 comprising a heat exchanger disposed in thesecond refrigerant connection between the pumping device and theevaporator, the heat exchanger lowering the temperature of therefrigerant in the second refrigerant connection.
 18. The motor coolantsystem of claim 13 wherein the pumping device lowers the density of therefrigerant within the motor housing to reduce windage losses of themotor.
 19. The motor coolant system of claim 13 wherein the pumpingdevice comprises one of an aerodynamic compressor or a positivedisplacement compressor.
 20. The motor coolant system of claim 19wherein the pumping device comprises one of a screw compressor, areciprocating compressor, a scroll compressor, or a vane typecompressor.
 21. The motor coolant system of claim 13 wherein the liquidexpander comprises one of an eductor, positive displacement expander ora turbine centrifugal expander.
 22. A method for cooling a motor of achiller system comprising the steps of: providing a first connectionwith a refrigerant loop, the first connection being configured toreceive refrigerant from an evaporator; providing a second connectionwith the refrigerant loop, the second connection being configured toreturn refrigerant to the evaporator; providing a motor in a motorcavity, the motor cavity being connected to the first connection and thesecond connection; circulating refrigerant from the first connectionthrough the motor cavity to the second connection with a pumping device;powering the pumping device with energy of expansion from a liquidexpander, the liquid expander being configured to expand refrigerant inthe refrigerant loop between a condenser and the evaporator; and whereinthe circulation of refrigerant in the motor cavity by the pumping devicecools the motor and lowers a pressure and gas density of a refrigerantin the motor cavity thereby reducing windage losses of the motor. 23.The method of claim 23 further comprising the step of connecting thepumping device and the liquid expander by one of an electricalconnection or a mechanical connection.
 24. The method of claim 24wherein the pumping device and liquid expander are combined as a singleunit.
 25. The method of claim 23 further comprising the step of coolingthe refrigerant in the second connection with a heat exchanger.
 26. Themethod of claim 26 wherein the heat exchanger uses a cooling liquid forthe condenser to cool the refrigerant in the second connection.